diff --git a/README.md b/README.md
index f60b163..1466ba1 100644
--- a/README.md
+++ b/README.md
@@ -256,6 +256,8 @@ You can then use the baton command in your workflow.
 
 The sync-test action tests syncing, granting, and revoking for a baton connector.
 
+Before the grant/revoke cycle it also syncs once with deliberately invalid credentials and asserts the connector exits with gRPC `Unauthenticated` (16) or `PermissionDenied` (7). That mapping requires `exit.LogExit` from baton-sdk v0.25.0+; connectors that still `os.Exit(1)` skip this check. Dummy values default to `invalid` for credential-like `BATON_*` env vars (tokens, passwords, secrets, keys). Override with `bad-credentials` when the value must parse as JWT, PEM, JSON, or similar before the connector will attempt authentication.
+
 ```yaml
 - name: Test Connector Sync
   uses: ConductorOne/github-workflows/actions/sync-test@v4
@@ -265,6 +267,9 @@ The sync-test action tests syncing, granting, and revoking for a baton connector
     baton-principal: "user123"
     baton-principal-type: "user"
     sleep: 2 # optional, wait 2 seconds after each write operation
+    # optional, only needed when dummy credentials must be well-formed:
+    # bad-credentials: |
+    #   BATON_PRIVATE_KEY=-----BEGIN PRIVATE KEY----- invalid -----END PRIVATE KEY-----
 ```
 
 ### Account Provisioning Test
diff --git a/actions/sync-test/action.yaml b/actions/sync-test/action.yaml
index bfa1d2a..c336472 100644
--- a/actions/sync-test/action.yaml
+++ b/actions/sync-test/action.yaml
@@ -29,6 +29,18 @@ inputs:
       Example:
         required-secrets: BATON_NETLIFY_ACCESS_TOKEN
     required: false
+  bad-credentials:
+    description: |
+      Env-file of invalid credentials for the unauthenticated sync check.
+      Each line is NAME=value. When omitted, credential-like BATON_* env vars
+      (and names listed in required-secrets) are set to "invalid". Override
+      when dummy credentials must be well-formed (JWT, PEM, JSON key, etc.).
+
+      Example:
+        bad-credentials: |
+          BATON_TOKEN=invalid
+          BATON_PRIVATE_KEY=-----BEGIN PRIVATE KEY----- invalid -----END PRIVATE KEY-----
+    required: false
 
 runs:
   using: "composite"
@@ -36,6 +48,14 @@ runs:
     - name: Download Baton
       uses: ConductorOne/github-workflows/actions/get-baton@v4
 
+    - name: Auth Error
+      env:
+        BATON_CONNECTOR: ${{ inputs.connector }}
+        REQUIRED_SECRETS: ${{ inputs.required-secrets }}
+        BAD_CREDENTIALS: ${{ inputs.bad-credentials }}
+      run: ${{ github.action_path }}/auth-error.sh
+      shell: bash
+
     - name: Grant Revoke
       env:
         BATON_CONNECTOR: ${{ inputs.connector }}
diff --git a/actions/sync-test/auth-error.sh b/actions/sync-test/auth-error.sh
new file mode 100755
index 0000000..35fcd03
--- /dev/null
+++ b/actions/sync-test/auth-error.sh
@@ -0,0 +1,150 @@
+#!/bin/bash
+
+# Sync with deliberately invalid credentials and assert the connector exits
+# with a gRPC status code: Unauthenticated (16) or PermissionDenied (7).
+#
+# Connectors must call exit.LogExit (baton-sdk v0.25.0+) to surface those
+# codes. The older main.go pattern always os.Exit(1); treat that as "not
+# migrated yet" and skip rather than fail.
+
+set -euo pipefail
+
+GRPC_UNAUTHENTICATED=16
+GRPC_PERMISSION_DENIED=7
+DEFAULT_INVALID_CREDENTIAL="invalid"
+
+if [ -z "${BATON_CONNECTOR:-}" ]; then
+  echo "BATON_CONNECTOR not set"
+  exit 1
+fi
+
+# Returns 0 if NAME looks like a connector credential env var.
+# Only BATON_* names are considered here; required-secrets and
+# bad-credentials can name additional vars.
+is_credential_var() {
+  local name="$1"
+  case "$name" in
+    BATON_CONNECTOR | BATON_ENTITLEMENT | BATON_PRINCIPAL | BATON_PRINCIPAL_TYPE | BATON)
+      return 1
+      ;;
+  esac
+  case "$name" in
+    BATON_*) ;;
+    *) return 1 ;;
+  esac
+  # Connection/config, not a secret.
+  case "$name" in
+    *_URL | *_ENDPOINT | *_PATH | *_HOST | *_ADDRESS | *_TIMEOUT | *_LEVEL | *_FILE | *_DIR)
+      return 1
+      ;;
+  esac
+  # Identifiers that often sit next to secrets.
+  case "$name" in
+    *_KEY_ID | *_CLIENT_ID | *_TENANT_ID | *_ORGANIZATION_ID | *_ORG_ID)
+      return 1
+      ;;
+  esac
+  case "$name" in
+    *TOKEN* | *PASSWORD* | *SECRET* | *KEY* | *CREDENTIAL* | *PAT | *PASSWD* | *BEARER*)
+      return 0
+      ;;
+  esac
+  return 1
+}
+
+# Collect unique env var names to overwrite, preserving insertion order.
+names=()
+seen=" "
+
+add_name() {
+  local name="$1"
+  case "$seen" in
+    *" $name "*) return 0 ;;
+  esac
+  seen+="$name "
+  names+=("$name")
+}
+
+while IFS= read -r name; do
+  [ -z "$name" ] && continue
+  if is_credential_var "$name"; then
+    add_name "$name"
+  fi
+done < <(compgen -e | LC_ALL=C sort)
+
+if [ -n "${REQUIRED_SECRETS:-}" ]; then
+  for name in ${REQUIRED_SECRETS//,/ }; do
+    [ -z "$name" ] && continue
+    add_name "$name"
+  done
+fi
+
+if [ ${#names[@]} -eq 0 ] && [ -z "${BAD_CREDENTIALS:-}" ]; then
+  echo "::notice title=auth-error test skipped::no credential env vars found to invalidate; skipping unauthenticated sync test"
+  exit 0
+fi
+
+if [ ${#names[@]} -gt 0 ]; then
+  for name in "${names[@]}"; do
+    export "${name}=${DEFAULT_INVALID_CREDENTIAL}"
+  done
+fi
+
+# Overlay caller-supplied dummy credentials. Used when a value must parse
+# as JWT, PEM, JSON, etc. before the connector will attempt authentication.
+if [ -n "${BAD_CREDENTIALS:-}" ]; then
+  while IFS= read -r line || [ -n "$line" ]; do
+    line="${line%$'\r'}"
+    # trim leading whitespace
+    line="${line#"${line%%[![:space:]]*}"}"
+    [ -z "$line" ] && continue
+    [ "${line:0:1}" = "#" ] && continue
+    case "$line" in
+      *=*) ;;
+      *)
+        echo "bad-credentials line is not NAME=value: $line"
+        exit 1
+        ;;
+    esac
+    name="${line%%=*}"
+    value="${line#*=}"
+    if [ -z "$name" ]; then
+      echo "bad-credentials line is missing NAME: $line"
+      exit 1
+    fi
+    export "${name}=${value}"
+    add_name "$name"
+  done <<< "$BAD_CREDENTIALS"
+fi
+
+if [ ${#names[@]} -eq 0 ]; then
+  echo "::notice title=auth-error test skipped::no credential env vars found to invalidate; skipping unauthenticated sync test"
+  exit 0
+fi
+
+echo "Syncing with invalid credentials in: ${names[*]}"
+
+set +e
+"$BATON_CONNECTOR"
+exit_code=$?
+set -e
+
+case "$exit_code" in
+  "$GRPC_UNAUTHENTICATED" | "$GRPC_PERMISSION_DENIED")
+    echo "Connector exited $exit_code as expected for invalid credentials."
+    exit 0
+    ;;
+  1)
+    echo "::notice title=auth-error test skipped::connector exited 1; unauthenticated sync test requires exit.LogExit (baton-sdk v0.25.0+). Skipping until the connector is updated."
+    exit 0
+    ;;
+  0)
+    echo "Connector succeeded (exit 0) with invalid credentials; expected Unauthenticated (16) or PermissionDenied (7)."
+    exit 1
+    ;;
+  *)
+    echo "Connector exited $exit_code with invalid credentials; expected Unauthenticated (16) or PermissionDenied (7)."
+    echo "If dummy credentials failed config validation, set the bad-credentials input to well-formed invalid values."
+    exit 1
+    ;;
+esac